1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the optical scanning apparatus, and is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which performs an electrophotography process.
2. Description of the Related Art
Conventionally, there are proposed various optical scanning apparatuses used for image forming apparatuses such as a laser beam printer and a digital copying machine. In these optical scanning apparatuses, a diverging beam emitted from a light source unit such as a semiconductor laser is guided to a deflection surface of a deflection unit such as a polygon mirror (optical deflector) by an incident optical system formed of a collimator lens, a cylindrical lens, and the like.
In this case, the diverging beam emitted from the light source unit is converted into a substantially collimated beam by the collimator lens or the like. Then, in order to compensate an optical face tangle error of the deflection surface, the beam converted into the substantially collimated beam is caused to form a line image on the deflection surface or in a vicinity thereof by a cylindrical lens having refractive power only in a sub-scanning direction. Then, the beam deflected by the deflection surface of the deflection unit is condensed by an imaging optical system having fθ characteristics, and by rotation action of the deflection unit, a light spot scans a drum surface of a photosensitive member as a surface to be scanned at substantially constant speed so as to form image information on the photosensitive drum surface.
Many optical scanning apparatus have the following structure. The imaging optical system includes a first lens unit having refractive power mainly in a main scanning direction, and a second lens unit having refractive power mainly in the sub-scanning direction. Further, the deflected beam from the deflection unit forms a spot on the surface to be scanned. In addition, in a sub-scanning section, the deflection surface and the surface to be scanned have a substantially conjugate relationship. Thus, a spot forming position shift in the sub-scanning section due to a deflection surface tangle error (optical face tangle error) is reduced. In other words, the imaging optical system constitutes an optical face tangle error compensation optical system.
Conventionally, as to such the optical scanning apparatuses, there is known an optical scanning apparatus in which design optical face tangle error performance is improved and a field curvature in the sub-scanning direction (sub-scanning field curvature) is reduced (U.S. Pat. No. 7,561,319).
The optical scanning apparatus described in Japanese Patent Application Laid-Open No. 2009-14953 discloses a technology for reducing both a sub-scanning field curvature and an imaging position shift (pitch unevenness) due to a geometric optical face tangle error of the deflection surface simultaneously. In the method of compensating the pitch unevenness due to the geometric optical face tangle error of the deflection surface, if the optical face tangle error occurs, it may be difficult to reduce both the sub-scanning field curvature and the gravity position shift of a spot intensity (amplitude) distribution on the surface to be scanned. Therefore, it is necessary to determine design performance from a wave optics viewpoint instead of a geometric method.